1. Field of the Invention
The present invention relates to a monopulse radar system, and more specifically to a monopulse radar system for radiating a signal from a transmitting antenna, and receiving a signal reflected by a target with a plurality of receiving antennas to thereby detect an angle of an existing position of the target.
2. Description of the Related Art
In a radar system, a monopulse system is known as one method of detecting an azimuth angle to a target. The monopulse system radiates a signal from a transmitting antenna and receives a signal reflected by a target through the use of two or more receiving antennas. At this time, information about the angle of the target to the radar system is obtained from the difference in amplitude or phase between the signals received by the individual antennas. In general, one using the phase difference is called a xe2x80x9cphase monopulsexe2x80x9d, whereas one using the amplitude difference is called an xe2x80x9camplitude monopulsexe2x80x9d. As this type of monopulse radar system, a radar system using a phase monopulse has been described in, for example, Japanese Patent Laid-Open No. H11 (1999)-271433.
The principle of operation of a monopulse radar system will be explained below using FIG. 2. A signal generated from a signal generator OSC1 is radiated or emitted from a transmitting antenna ANT1. The radiated signal is reflected by a target, which in turn is received by two receiving antennas ANT2 and ANT3. The received signals are converted into low frequency signals by mixers MIX1 and MIX2, followed by being subjected to signal analytic processing such as FFT (Fast Fourier Transform) in signal processing means P1.
In the case of the phase monopulse, the difference in phase between signals received by two antennas is determined. When the two receiving antennas are laid out with an interval d defined therebetween as shown in FIG. 3, the following equation (1) is established between the difference in phase between signals received by them, and an azimuth angle to a target:
xcex94xcfx86=(2xcfx80d/xcex)sin xcex8xe2x80x83xe2x80x83(1)
On the other hand, when the amplitude monopulse is used, directivity S1 of the receiving antenna ANT2 and directivity S2 of the receiving antenna ANT3 are distributed as represented in a gain characteristic of FIG. 4. A signal strength based on the sum of the signals received by the two antennas and a signal strength based on the difference between the signals are respectively represented as gain characteristics shown in FIG. 5. Further determining the ratio between the two from the sum signal and the difference signal results in a gain characteristic R1 of FIG. 6. The magnitude of this ratio and angles are associated with each other to thereby determine each corresponding angle.
An in-vehicle radar system often makes use of an ultra-high frequency like a millimeter wave. Frequencies ranging from 76 GHz to 77 GHz are assigned to a vehicle radar used in a vehicle-to-vehicle distance warning system for a vehicle. In general, parts used in such an ultra-high frequency are expensive and large in part-to-parts characteristic variations as compared with parts used in a low frequency. Further, since the wave length is very short in the ultra-high frequency like the millimeter wave, variations in characteristic occur even upon assembly of modules for the radar system.
On the other hand, the conventional monopulse radar system shown in FIG. 2 detects a phase difference or an amplitude difference from the low-frequency signals produced by the mixers MIX1 and MIX2 and calculates the azimuth angle of the target, based on it. Thus, it is necessary to grasp whether a channel CH1 based on the receiving antenna ANT2 and a channel CH2 based on the receiving antenna ANT3 coincide in characteristic with each other with very high accuracy, or the difference therebetween in advance.
In the radar system using signals lying in a millimeter wave band, however, a phase error or an amplitude error is developed between channels due to the aforementioned reason, and an error is developed in a detected azimuth angle of target.
As a countermeasure taken against it, a method of detecting and selecting used parts in advance and using only ones matched in characteristic with each other might be adopted. However, this will result in an increase in part cost. Further, variations developed upon part assembly cannot be eliminated.
As another countermeasure, there is known a method of actually radiating a radio wave from the forward of a radar system and correcting a phase difference or an amplitude error developed between channels, based on a detected signal obtained therefrom as has been described in, for example, Japanese Patent Laid-Open No. H5 (1993)-232215. In such a method, however, the system becomes large-scale and takes a lot of trouble over its calibration work. Therefore, the manufacturing cost thereof will increase due to the new addition of this work.
A problem arises in that any countermeasures referred to above do not take into consideration variations in characteristic after the shipment of each product, and the accuracy of a detected angle is deteriorated where variations in characteristic occur due to some kind of factors such as environmental variations, time variations in parts characteristic.
Accordingly, it is a main object of the present invention to provide a monopulse radar system capable of easily correcting a phase error or an amplitude error developed between receiving channels corresponding to a plurality of receiving antennas of the monopulse radar system and reducing a manufacturing cost thereof.
It is another object of the present invention to provide a monopulse radar system capable of coping even with characteristic variations after product shipment such as environmental variations, time variations in parts characteristic and correcting a phase error and an amplitude error.
In order to achieve the above object, a monopulse radar system of the present invention is provided with a calibration signal transmission line for supplying part of a transmit signal to each of respective receiving antennas or each of signal transmission lines (i.e., each of individual output parts of a plurality of receiving antennas) connected to the receiving antennas as a signal for calibration. The monopulse radar system is also provided with correcting means for calculating an azimuth angle of a pseudo target based on the calibration signal, determining an azimuth angle to be originally calculated by a monopulse radar, using the azimuth angle of the pseudo target, and adjusting at least either amplitude or phase so that the azimuth angle to be originally calculated and the azimuth angle of the pseudo target calculated precedently by each calibration signal coincide with each other.
In a preferred embodiment of the present invention, the signals for calibration respectively applied to the output parts of the plurality of receiving antennas are set as being equipower and equiphase. Particularly when directivities of the two receiving antennas are made symmetrically with respect to a central direction, such a state that the signals are inputted to the respective antennas at equal power and in equiphase fashion in this way, occurs when a target is placed on a center line indicative of the center of an angle to be detected by a radar. Thus, a correcting process is performed in such a manner that the azimuth angle of the pseudo target, which is calculated from the signals for calibration, extends on the center line of the radar.
The signal for calibration is used where the signal is always supplied to the output parts of the plurality of receiving antennas, and where a switch is provided on a calibration signal transmission line for supplying the signal calibration and a switch driving unit for controlling conducting and cut-off states of the switch is provided. As the control on the conducting and cut-off states, there are one for normally bringing the calibration signal transmission line to the cut-off state and bringing it to the conducting state upon calibration, and one for generating the conducting state of the switch periodically or with predetermined timing. In particular, a signal processing device such as a DSP is used to perform either one of the above-described calibrations automatically or with arbitrary timing, thereby correcting variations in characteristic developed after product shipment due to environmental variations, time variations in parts characteristic, etc. In the present embodiment, the accuracy of the angle detected by the monopulse radar can be maintained by ever-execution of the calibration.
The correcting means of the monopulse radar according to the present invention comprises signal processing means for calculating an azimuth angle of a pseudo target, based on the signal for calibration, determining an azimuth angle to be originally calculated by the monopulse radar through the use of the azimuth angle of the pseudo target, determining correction data so that the azimuth angle to be originally calculated and the azimuth angle of the pseudo target, which is precedently detected by the signal for calibration, coincide with each other, writing the correction data into memory means, and correcting an azimuth angle of a target detected when actually activated as a radar, according to signal processing on the basis of the correction data stored in the memory means.
As another preferred embodiment, the signal for calibration is modulated with a low frequency signal. Thus, the calibration signal is superimposed on a reflected signal, thus causing inconvenience. Namely, if the operation of an antenna is not perfectly stopped upon calibration, then a signal radiated from a transmitting antenna is reflected by an object present ahead of the antenna, which is then received by its corresponding receiving antenna depending on environments under which calibration is made. The present embodiment prevents such inconvenience that when a transmit signal is used as a signal for calibration as it is, this signal is superimposed on the calibration signal, so that accurate calibration cannot be performed. The present embodiment is also effective in preventing a low frequency signal converted by a mixer from being brought into a DC current in a homodyne type radar wherein the same ones are used for a local signal generator for generating a transmit signal and a signal generator for generating a local signal supplied to the mixer.